Geraint and Emily secretly together (2,6)
I believe the answer is:
in tandem
'together' is the definition.
(working in tandem is working together)
'geraint and emily secretly' is the wordplay.
'secretly' says the answer is hidden in the clue.
'IN TANDEM' is hidden within 'geraint and emily'.
(Other definitions for in tandem that I've seen before include "Manned it as twosome" , "Together - as on a bicycle made for two" , "Working together" , "Together (as a pair)" , "One behind the other or alongside" .)